1. Field of the Invention
The invention relates to thermoplastic adhesives and more particularly to thermoplastic-polyimide (TPI) adhesive lamination films and coatings.
2. Description of the Prior Art
Thermoset adhesives are the dominant bonding technology in most electronic applications, generally because they are relatively inexpensive and easy-to-use. They become very strong through polymer cross-linking, usually at elevated temperatures. The resultant bondline is generally quite brittle. Thermoset adhesives in electronics include epoxy, acrylic and silicone.
Thermoplastic adhesives are used in applications where a ductile bondline is required. Their polymer chains are very stable, and do not require cross-linking Therefore, the resultant bondline has ductility. It will not crack under stress and strain.
Polyimide polymer provides the ultimate in thermal, chemical and structural durability as opposed to lower temperature thermoplastic polymers such as, for example, polyethylene, polypropylene, polyester, and polyamide. Most polyimide polymers are made through a condensation reaction within the polymer chain, where a water molecule is eliminated during the closure of a carbon ring. Carbon rings within the aromatic polymer chain are the basis for polyimide polymer stability. The precursor polymer is polyamic acid.
Thermoplastic-polyimide (TPI) adhesive provides an exceptional bondline for demanding electronic packaging applications, since it is very durable even at minimal thicknesses. This is especially critical where two different layers with dissimilar Coefficients-of-Thermal-Expansion (CTE) are bonded together and undergo repeated thermal cycles. This is a leading cause of failure in many structures, electronic and otherwise. The ability of TPI adhesive to be ductile and strong make it ideal for thermal-interface bondlines where the need to withstand CTE induced stress as well as a minimal thickness to maximize thermal transfer between metal layers, such as between a heat generating device and a heat sink, is required.
TPI adhesive laminate films are known in the prior art and are commercially available from, for example, DuPont Corporation, Wilmington Del., under the trade names Pyralux® AP and Kapton® KJ and LJ films; these films are solvent-cast, as is this invention. In addition, TPI adhesive films can be extruded from SABIC's (Riyadh, Saudi Arabia) Ultem® and Extern® resins and DuPont's Aurum® resin. In all of those, as best known to the applicant, the TPI adhesive is in a fully cured state, i.e., the precursor polymer has been substantially converted to TPI and the process solvents have been evaporated. Use of these requires relatively high temperatures and pressures. In contrast, the under cured or B-staged films and coatings of the present invention can be employed and used in manufacturing operations at moderate conditions approaching those of conventional thermoset systems. This is accomplished by engineering the drying and curing level of the thermoplastic-polyimide adhesive layer of the film in a way that facilitates handling and use of the product. Partially curing the TPI adhesive allows lamination processing at much lower temperatures and pressures than a fully cured TPI adhesive layer.
Under curing, or B-staging, thermoplastic-polyimide adhesive allows bondline processing at much lower temperatures and pressures than conventional fully cured TPI coatings: the reduction in temperature can be as much as 80° C. or more; the reduction in pressure can be as much as 95-99%. The B-staging process for these solvent-cast TPI systems is much different than conventional thermoset adhesive systems such as the high-solids epoxy and silicones.
TPI polymers in their precursor polyamic-acid (PAA) liquid form are a relatively low-solids solution (<20%) in a strong solvent, such as NMP, DMAc or DMF. NMP or n-pyrol is generally preferred as it is less hazardous than other equivalent solvents, and when volatilized, evacuates bondlines quickly and smoothly, without blistering. The PAA liquid solution is applied as a thin layer to a substrate, by any of a variety of conventional coating methods. Heat is applied to the coating through a conventional coating oven with hot air, which evaporates off the solvent at a controlled rate and can start the conversion of the PAA to polyimide (PI). In this invention, only enough heat, hot air, is used to evaporate a controlled amount of solvent and only a minor conversion, if any, of PAA polymer to PI polymer occurs. Leaving residual solvent in the B-staged coating dramatically increases the flow of polymer when the coating is reheated during the lamination process. This is critical in adhesive bondlines where micro-irregular surfaces are being mated. Practically all non-mirror surfaces are very irregular on a micro-level. In addition, the PAA polymer has hydroxyl groups which facilitate adhesion of the polymer to adjacent surfaces. As shown in FIG. 1 below, the schematic shows the condensation reaction of PAA polymer, made from PMDA/ODA monomers, to generic “Kapton” PI polymer.
It is important to delineate the concept of traditional curing of PAA polymer coatings to PI, as our invention represents a significant departure. In prior art, PAA coatings are exposed to very high heat during the coating process, removing all the solvent and converting much or all of the PAA to PI. This process removes practically all of the polymer flow and reactivity characteristics, so such a TPI lamination would require extremely high temperatures and pressures. Traditional lamination generally requires a very high-temperature platen press or vacuum autoclave. A totally cured TPI adhesive would need to be melted to provide bonding to adjacent surfaces; this would generally occur at 320-350° C. In addition, the fully cured TPI would require immense pressure applied during melting to flow the extremely viscous polymer into the adjacent surface to ensure adhesion.
The representative process conditions for coating and laminating for a representative TPI adhesive polymer are shown in FIG. 2 below, for both a prior art state and the B-staged state of the invention. Other TPI polymers could have different absolute temperatures and pressures for processing, but the relative differences between traditional cure and B-staged states would be about the same as the example shown. Coating and employment of the film in laminating procedures are performed sequentially, with different processes and generally at different locations.